Joining Tesla's "Gigafactory" battery project would raise investment risks, Panasonic President Kazuhiro Tsuga, told reporters Thursday at a briefing in Tokyo. Tesla, which announced plans for the facility in February, said it's reviewing potential sites in four southwestern U.S. states. The plant may require as much as $5 billion to build and employ about 6,500 people by 2020, the Palo Alto-based company said.

Musk, who is also Tesla's co-founder and biggest investor, has said Panasonic may be involved in the factory. However, the Osaka, Japan-based company's participation is "not 100 percent confirmed," he told Bloomberg Television last month.

"Having Panasonic as a joint venture partner would facilitate strategic access to Pansonic's supply chain, and reduce risks," Craig Irwin, a New York-based analyst for Wedbush Securities, wrote in a note Thursday.

Irwin, who has an outperform rating on the stock, trimmed his target price for Tesla's shares to $275 from $295.

Texas, Arizona, New Mexico and Nevada, the four states Tesla identified as possible factory locations, have started lobbying efforts to win the plant. Tesla has raised about $2 billion in a convertible bond sale to help fund the plant and for product development.

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Musk, 42, has said the plant is critical to helping Tesla drive down battery costs and to expand lithium-ion cell supplies to allow the carmaker to sell hundreds of thousands of electric vehicles annually. Tesla also wants to make batteries for home storage of electricity generated by solar panels, allowing customers to reduce use of power from utilities.

Tesla relies on Panasonic for the battery materials, said Menahem Anderman, president of Total Battery Consulting Inc., in Oregon House, California. If Panasonic isn't involved, that will make the factory much more of a challenge, he said.